There
were several different varieties of eunuchs:
Sandali, or clean-shaven: The parts are swept off by a single cut
of a razor, a tube (tin or wooden) is set in the urethra, the wound
is cauterized with boiling oil, and the patient is planted in a
fresh dung-hill. His diet is milk, and if under puberty he often
survives.

The eunuch whose penis is removed: He retains all the power of
copulation and procreation without the wherewithal; and this, since
the discovery of caoutchouc, has often been supplied.
The eunuch, or classical thlibias and semivir, who has been rendered
sexless by the removing of the testicles..., or by their being bruised...,
twisted, seared or bandaged.

Black eunuchs tended to be of the first category: Sandali, while
white eunuchs were of the second or third categories, thus have
part or their entire penis intact. Because of their lack of parts,
black eunuchs served in the harem, while white eunuchs served in
the government (and away from the women). At the height of the Ottoman
Empire, as many as six to eight hundred eunuchs served within the
Seraglio (palace). Most eunuchs arrived as gifts from governors
of different provinces. At the end of their training as young eunuch
pages, eunuchs were assigned to service. White eunuchs were placed
under the patronage of various government officials or even into
the service of the Sultan himself (like in Topkapi Palace). If they
were black eunuchs, they were placed into the service of a harem
personage, such as a Kadin, or a daughter or sister of the Sultan.
They could also serve under the Kizlar Agha (master of the girls),
the Chief Black Eunuch.